1. Field of the Invention
The present invention relates in general to a recording head for recording or printing images such as characters and graphical representations, by applying an electric current to a recording medium, or a ribbon or film or other form of intermediate member interposed between the recording medium and the recording head. More particularly, the invention is concerned with the configuration of a distal end portion of such a recording head at which the head contacts the recording medium or intermediate member.
2. Discussion of the Prior Art
There will be first described prior art recording heads, and problems encountered in these heads. Various types of recording heads for recording by application of an electric current to a recording medium or an intermediate member have been proposed up to the present. In particular, there is known a recording head having a laminar or multi-layer structure which includes a substrate or substrates, and an array of recording electrodes and an array of return circuit electrodes which are supported by or formed on the substrate or substrates. Examples of this type of recording head are disclosed in laid-open Publication Nos. 61-35972, 62-292461, 54-141140, 58-12790 and 61-230966 of unexamined Japanese Patent Applications.
There is known another type of recording head, in which the array of recording electrodes is formed on one of opposite major surfaces of a substrate, as disclosed in laid-open Publication Nos. 58-104787, 61-37493, 63-30279, 63-87264, 63-160855 60-78772 and 62-238767 of unexamined Japanese Patent Applications.
As disclosed in the publications identified above, the recording head of the types indicated above are adapted such that an electric current is applied to an electrically resistive or conductive layer formed or coated on or carried by a suitable recording medium or a suitable planar intermediate support member in the form of a sheet, film or ribbon. The electrically resistive or conductive layer may be formed on a roller or other support member, or constitute an inner layer of the recording medium or intermediate support member. In a recording operation by using an intermediate ribbon or film having an electrically resistive layer and an ink layer, for example, an electric current applied to the resistive layer through the recording head causes Joule heat to be generated by the resistive layer, whereby selected local areas of the ink layer are heated of the Joule heat, and the ink material in these heated local areas is fused, vaporized or diffused. As a result, the ink material is transferred to the appropriate local areas of the recording medium so as to form a black or colored image. If an electric current is applied directly to a recording medium, the appropriate local areas of the medium are suitably colored due to Joule heat generated by an electric current, or due to removal of the covering material from the medium surface due to an electrical discharge occurring thereon.
The electrically resistive layer provided on the recording medium or intermediate support member may be an electrically conductive layer, an electrically conductive or resistive ink layer (which serves also as an ink-bearing layer), a heat-sensitive layer having an electrolyte, or any form of layer through which an electric current may flow.
In a recording or printing operation by the recording head for use with the recording medium or intermediate support member as described above, the recording electrodes and the return circuit electrode or electrodes must be held in electrical contact with the electrically resistive layer of the recording medium or support member. To this end, the electrodes used in the known recording heads as disclosed in the publications indicated above are formed of a material which has a higher degree of wear resistance than the material of the substrate structure and an electrically insulating layer used for the heads.
An example of such a known recording head is partly illustrated in FIG. 23, wherein a multi-layer structure is formed by two substrates 21, an array of recording electrode 22 formed on one of the substrates 21, an array of return circuit electrodes 23 formed on the other substrate 21, and an electrically insulating layer 24 which separates the two arrays of electrodes 22, 23 and to which the electrode arrays 22, 23 are bonded by respective layers of a suitable adhesive 25, 25. In this arrangement, the end face of the substrate structure 21 occupies a considerably large portion of the entire contact face at the distal end of the head which is adapted to contact the surface of the electrically resistive layer of the recording medium or intermediate support member. In other words, the area of the contact end faces of the electrodes 22, 23 is comparatively small. This area ratio of the contact end face of the substrate structure 21 and electrodes 22, 23 does not provide for a desired electrical contact between the electrodes and the electrically resistive layer. When a recording operation requires a relatively large contacting force of the electrodes against the resistive layer, the known arrangement is not satisfactory for producing high-quality images.
For improving the electrical contact between the electrodes and the resistive layer of an ink ribbon for example, a recording head having a generally pointed contact end portion is proposed as shown in FIG. 24. While this recording head assures improved electrical contact between the electrodes and the resistive layer during an initial period of use, the ratio of the contact end face area of the substrate structure 21 with respect to that of the electrodes 22, 23 increases as the pointed end of the head wears. Satisfactory contacting of the electrodes with the resistive layer may not be obtained if the worn-out contact end of the head is ground for re-shaping by a relatively easy method while the head remains installed on the relevant recording apparatus.
An alternative known approach is to use the electrodes 22, 23 which have increased thicknesses, as indicated in FIG. 25. This arrangement, however, lowers the efficiency or ease of forming the electrodes in the desired patterns in the form of mutually spaced apart stripes.
Another type of recording head is illustrated in FIG. 26. This recording head has an array of recording electrodes 22 which is formed on one major surface of the substrate 21 and covered by an electrically insulating layer 27 made of a comparatively soft material. Although the contact of the electrodes 22 and insulating layer 27 with the resistive layer is better than the contact of the multi-layer heads indicated above, the contact end portion of the electrodes 22 comparatively rapidly wears since the head contacts the resistive layer at one edge thereof on the side of the electrode array 22. As the contact edge of the head is worn or ground for re-shaping, the area ratio of the contact end face of the substrate 21 with respect to the contact end face of the electrodes 22 increases, whereby the electrical contact of the electrodes is deteriorated during use of the head.
In the recording head including the electrodes for applying an electric current to an electrically resistive layer as described above, it is required that the distance between the recording electrode array and the return circuit electrode array be controlled to a small value as accurately as possible, for avoiding a crosstalk between the electrodes to thereby improve the printing result. To meet this requirement, the electrically insulating layer must be formed with a sufficiently small thickness. However, it is practically difficult to form the electrically insulating layer 24 with a small thickness, while maintaining a sufficient degree of mechanical strength of the layer. Thus, the arrangement in which the electrode-to-electrode distance is determined by the thickness of an interposed electrically insulating layer is not satisfactory for reducing the distance between the recording and return circuit electrodes 22, 23.
Further, the known multi-layered head arrangements as shown FIGS. 23 through 25 use the adhesive layers 25 for bonding together the two substrates 21, 21, with the electrically insulating layer 24 interposed between the opposite surfaces of the substrates 21, 21 on which the recording and return circuit electrode arrays 22, 23 are formed. In these arrangements, the distance between the recording and return circuit electrodes 22, 23 is influenced by the thickness of the adhesive layers 25, 25. Since the thickness of the adhesive layers 25 tends to be varied due to a variation in the amount of application of the adhesive material and/or different manners of the application, the electrode-to-electrode distance is likely to be varied and cannot be accurately controlled to a desired nominal value. In this respect, the known head arrangements of FIGS. 23-25 are not sufficiently satisfactory in the quality of printing.